Sandstone reservoirs containing significant amount of clays (30-40 wt. %) with moderate permeability (20-50 mD) provide a unique challenge to surfactant-based enhanced oil recovery (EOR) processes. A critical risk factor for these types of reservoirs is adsorption of surfactants due to greater surface area attributed to clays. Clays also have high cation exchange capacity and can release significant amounts of divalent ions that lead to increased retention of the surfactant. These factors can adversely affect the economics of a flood used for enhanced oil recovery.
In a chemical flood, surfactant cost impacts overall project economics. Most of the surfactant injected into a reservoir is lost due to adsorption on the reservoir rock surface. Sandstone reservoirs have a net negative charge at neutral pH. Therefore, anionic surfactants are preferred over nonionic or cationic surfactants because of their lower adsorption on the sandstone reservoir rock. When a reservoir brine contains significant amounts of divalent ions (i.e., Ca and/or Mg), the use of alkali may be prohibitive and cannot be used to reduce adsorption.
Certain compositions can be mixed with a surfactant blend (as “mitigation agents”) or can be injected ahead of the surfactant front (as “sacrificial agents”), or both, to satisfy the adsorption capacity of the reservoir rock and reduce surfactant adsorption by the rock. A sacrificial or mitigation agent should reduce the surfactant adsorption significantly to offset the added cost of the sacrificial agent. A sacrificial or mitigation agent should be relatively inert and should not influence crude oil/surfactant/brine interactions by causing a change in optimal salinity or interfacial tension. Known sacrificial or mitigation agents include, among others, sodium polyacrylate, glyceric acid, glycolic acid, sodium metaborate, lignin sulfonates, polyethylene glycol (PEG), and polypropylene glycol (PPG).
U.S. Pat. No. 4,005,749 describes a waterflooding process for oil recovery wherein a water-soluble polyalkylene oxide having a molecular weight of at least 1200 is used as a sacrificial agent to retard adsorption of the surfactant. Injection of the sacrificial agent is followed by an aqueous slug of a surfactant. In an alternative approach, the surfactant slug includes a polyalkylene oxide sacrificial agent. Among the reservoirs treated is a reservoir sand having a high clay content (about 18%). It is unclear from the 749 patent how addition of alkali would impact adsorption of the surfactant.
U.S. Pat. No. 4,452,308 describes a method of using a low molecular weight polyalkylene glycol as a sacrificial agent for a surfactant flooding process. The useful molecular weights taught are 200 to 1200. The references describes the “vexing” problems of surfactant loss due to adsorption onto formations and precipitation by polyvalent cations. The lower molecular weight polyethylene glycols are shown to be effective in reducing surfactant loss with bentonite clays, but not with kaolinite clays. Because the total dissolved solids content of the brines was high (94,000-96,000 TDS), the use of alkali was not considered.
DE 0090920 describes a process for recovering oil from an oil-bearing formation whereby an aqueous solution of a polyethylene glycol, a polypropylene glycol, or an EO-PO copolymer is injected before, during, or after introduction of an aqueous viscous solution of a polysaccharide, a hydroxyalkylcellulose, a hydrophilic polyacrylamide, or a hydrophilic vinyl acrylamide copolymer. The epoxide polymer is used to minimize adsorption onto the formation of the polysaccharide or other hydrophilic polymer. The impact of alkali is not discussed.
W. T. Osterloh et al., “Surfactant-Polymer Flooding with Anionic PO/EO Surfactant Microemulsions Containing Polyethylene Glycol Additives,” SPE/DOE 24151, Eighth Symposium on Enhanced Oil Recovery, Tulsa, Okla. (April, 1992) 485 describes results of experiments designed to test the efficacy of polyethylene glycols in reducing the adsorption of certain surfactants onto clay. The experiments were conducted in a high-salinity brine (190,000 mg/L TDS). The authors concluded that static adsorption of certain alcohol ether sulfate surfactant blends on kaolinite could be lowered to undetectable levels using PEG-1000 as a sacrificial agent, and that PEG-1000 was generally much more effective than PEG-400 or PEG-20M in reducing static adsorption of the surfactant on kaolinite.
The industry would benefit from the availability of improved methods for enhanced oil recovery. In particular, methods that maximize surfactant utilization in clay-rich sandstone formations are needed. Ideally, the methods could utilize aqueous solutions of sacrificial or mitigation agents to avoid the added complication of solids handling. A desirable method could be used in fresh-water environments where swelling of the clay aggravates surfactant adsorption, or in high-salinity, high divalent ion environments where alkali cannot be used.